A liquid crystal display (LCD) device is adapted to display an image by adjusting light transmittance of a liquid crystal using an electric field.
An example of a LCD device includes an LCD module for displaying an image in response to a video data signal in a system, and a pulse width modulation unit for outputting a control signal for control of a microcomputer provided in the system.
The control signal is input to the microcomputer to control the operation thereof. This control signal is an alternating current (AC) signal which alternately has a high-logic voltage and a low-logic voltage. It has a unique duty factor according to the role thereof.
That is, in order to enable the microcomputer to execute various operations, the control signal has various duty factors corresponding to the operations. The duty factor of the control signal is adjusted by a device such as the pulse width modulation (PWM) unit. In response to a command from the user, the PWM unit generates the control signal having a duty factor corresponding to the command and transfers the control signal to the microcomputer.
FIG. 1 is a schematic view illustrating a process of transferring the control signal from the PWM unit to the microcomputer.
As shown in FIG. 1, the control signal output from the PWM unit 186 is input to the microcomputer 196, via an impedance matching circuit 105 and a resistor R2. The impedance matching circuit 105 functions to perform impedance matching between the PWM unit 186 which outputs the control signal and the microcomputer 196 which receives the control signal, so as to prevent the control signal from the PWM unit 186 from being distorted when being inputted to the microcomputer 196. The impedance matching circuit 105 includes a resistor R1 connected between a transmission line 111 and a ground terminal GND, and a capacitor C1 connected in parallel with the resistor R1.
When the output from the PWM unit 186 is abnormal, the control signal may be distorted or have a excess voltage, resulting in damage to the microcomputer 196 which receives the abnormal control signal. That is, the control signal may be input to the microcomputer 196 in the form of an over-voltage signal whose level exceeds a maximum allowable voltage value, or an under-voltage whose level does not reach a minimum allowable voltage value.